Bias circuit for differential amplifier

ABSTRACT

A separate emitter follower type bias circuit is connected to each base electrode of a transistorized differential amplifier which handles unbalanced input signals. A series circuit of resistors is employed for supplying a suitable bias voltage common to both emitter follower type bias circuits.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a bias circuit, and moreparticularly to a bias circuit for a differential amplifier of the typeof a single drive.

2. The Prior Art

In some differential amplifiers having single-ended input circuits, thebase bias voltage is derived from an emitter-follower connected to thebases of both of the differentially connected transistors. Although thebase of the differentially connected transistor to which the inputsignal is not applied could be connected to a low impedance supplycircuit, the other differentially connected transistor, to which theinput signal is applied, must have a relatively high impedance basecircuit so as not to attenuate the input signal. The relatively highimpedance circuit requires that there be a resistor between the base ofthe signal input transistor and the emitter of the emitter-followercircuit that supplies the base bias voltage. Base current flowingthrough that resistor affects the bias voltage at the base.

Balanced operation of the two differentially connected transistorsrequires that the other of the two transistors also have a resistorbetween its base and the emitter of the emitter-follower circuit. Inorder to hold the base of the second differentially connected transistorsubstantially at ground level, a capacitor may be connected between thatbase and ground. However, this is not satisfactory for integratedcircuit use because of the necessity to provide a connection for thecapacitor. Furthermore, it affects the low frequency response of thedifferential amplifier.

Balancing resistors may be inserted in the emitter leads of thedifferentially connected transistors instead of connecting the base ofthe second transistor to ground through a capacitor. However, thesebalancing resistors reduce the gain that may be obtained from thedifferential amplifier.

SUMMARY OF THE INVENTION

It is one of the objects of the present invention to provide an improvedemitter-follower bias circuit for a differential amplifier handlingunbalanced input signals, or a differential amplifier having asingle-ended input circuit.

Other objects and advantages of the invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings.

In accordance with the present invention separate emitter-follower biascircuits are connected to the base electrodes of the transistors of adifferential amplifier. The separate emitter-follower circuits have biascircuits of their own that equalize the base bias voltage applied to thedifferential amplifier transistors by way of the emitter-followercircuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic diagrams showing differential amplifierswith common emitter follower base bias circuits.

FIGS. 3 and 4 are schematic diagrams showing embodiments of differentialamplifier circuits with separate emitter follower base bias circuitsaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A differential amplifier 10 having an unbalanced input is shown inFIG. 1. In this circuit the emitter electrodes of a pair of transistors11 and 12 are connected directly together to a common constant currentsource 13. The base electrode of the transistor 11 is connected to aninput terminal 14, the collector electrodes of the transistors 11 and 12are connected through their respective load resistors 16 and 17 to apower supply terminal 18 having a voltage +V_(cc). Two output terminals19 and 21 are connected to the collector electrodes of the transistors11 and 12, respectively. A transistor 22 connected in an emitterfollower circuit provides base bias voltage for the differentialamplifier 10. The emitter of the transistor 22 is connected by resistors23 and 24 to the base electrodes of the transistors 11 and 12 to applythe emitter voltage of the transistor 22 as base bias voltage to thetransistors 11 and 12 respectively.

In this circuit, if the resistor 23 is not provided but, instead, thebase of the transistor 11 is connected directly to the emitter of thetransistor 22, an input signal from the input terminal 14 will beby-passed through the emitter electrode of the transistor 22, whichforms a low impedance path, and hence the practical voltage gain of theamplifier 10 will be lowered. To avoid this, the resistor 23 isprovided, but base current of the transistor 11 flowing through theresistor 23 causes a voltage drop. As a result, the base voltage of thetransistor 11 is lowered by that voltage drop.

In order to keep the operating conditions of the transistors 11 and 12equal, the base voltage of the transistor 12 is reduced by connectingthe resistor 24 in series between the emitter of the transistor 22 andthe base of the transistor 12. For this reason, in general, theresistors 23 and 24 are selected equal in resistance value.

Since the resistor 23 is provided to avoid reduction of the input signalat the input terminal 14, and since the transistor 11 is connecteddifferentially and its input impedance is rather high, the resistor 23is required to have a high resistance value. If the resistance value ofthe resistor 23 is selected to be high, the resistance value of theresistor 24 must be made high, also, corresponding to the resistor 23.However, if the resistance value of the resistor 24 is high, theequivalent base resistance of the transistor 12 becomes high. As aresult, the amplifier 10 cannot operate as an ideal differentialamplifier. Since the differential amplifier 10 has a single-ended input,the base of the transistor 12 must be operated at AC ground. However, asthe resistance value of the resistor 24 becomes higher, the baseelectrode of the transistor 12 becomes farther removed from AC ground.As a result, input signal components appear at the base electrode of thetransistor 12 in phase with signals at the base electrode of thetransistor 11, which reduces the voltage gain of the amplifier 10.

To avoid this, even when the resistance values of the resistors 23 and24 are high, and the base electrode of the transistor 12 may beby-passed to ground through a capacitor 26 shown in FIG. 1 in dottedlines. However, such a capacitor 26 is not preferred if the circuit isformed as an integrated circuit.

Therefore, when the differential amplifier 10 shown in FIG. 1 is made asan integrated circuit, the resistance value of the resistor 23 is madegreat, that of the resistor 24 is made small, and resistors 27 and 28are connected between the emitter electrodes of the transistor 11 and 12and the constant current source 13, respectively, as shown in FIG. 2 inwhich reference numerals which are the same as those used in FIG. 1 showthe same elements. In this case, if it is merely selected that theresistance value of the resistor 23 is great while that of the resistor24 is small, the base voltage of the transistor 11 becomes differentfrom that of the transistor 12, and accordingly an offset voltage isproduced at the output terminals 19 and 21. Thus, when the differencevoltage between the base voltages of the transistors 11 and 12 becomesmore than several ten millivolts (mV), the differential amplifier 10shown in FIG. 1 is not a linear amplifier and tends to operate in acurrent switch mode. To avoid this defect, when the resistors 23 and 24differ in resistance value, the resistors 27 and 28 are provided, asshown in FIG. 2, so as to prevent the occurrence of the offset voltageat the terminals 19 and 21 and to operate the differential amplifier 10as a linear amplifier.

However, with the circuit shown in FIG. 2 in which the resistors 27 and28 are provided, if it is assumed that the resistance value of theresistors 27 and 28 is taken as R_(E) and that of the resistors 16 and17 as R_(L), the voltage gain of the differential amplifier 10 shown inFIG. 2 is expressed as R_(L) /R_(E). When the circuit shown in FIG. 2 isformed as an integrated circuit, the resistance value R_(L) of theresistors 16 and 17 is restricted and hence the voltage gain isrestricted. As a result, a great voltage gain cannot be obtained.

An embodiment of the present invention will be now described withreference to FIG. 3 in which the elements corresponding to those used inFIGS. 1 and 2 are identified by corresponding reference numerals.

In the embodiment of FIG. 3, resistors 29 and 31 are connected in seriesbetween the voltage source terminal 18 to which a voltage of +V_(cc) issupplied and the ground. The connection point between the resistors 29and 31 is connected to the base electrode of the transistor 22 to supplya bias voltage to the same. The collector electrode of the transistor 22is connected to the voltage source terminal 18, and its emitterelectrode is grounded through a resistor 32. Thus, the transistor 22 isconnected in an emitter follower configuration. The emitter electrode ofthe transistor 22 is further connected through a resistor 33 to the baseelectrode of the transistor 11 to supply the emitter voltage of theformer to the base electrode of the latter as a bias voltage thereof.

In this invention, as shown in FIG. 3, there is further provided atransistor 34 whose base electrode is connected through a resistor 36 tothe connection point between the resistors 29 and 31 or to the baseelectrode of the transistor 22, the collector electrode of thetransistor 34 is connected to the voltage source terminal 18, and itsemitter electrode is grounded through a resistor 37 to make thetransistor 34 as an emitter follower configuration. The emitterelectrode of the transistor 34 is connected further to the baseelectrode of the transistor 12 to supply the emitter voltage of theformer to the base electrode of the latter as its bias voltage.

With the circuit construction described above with reference to FIG. 3,if it is assumed that

V_(BE22) and V_(BE34) represent the base-emitter voltages of thetransistors 22 and 34, respectively;

V_(BB) represents the base voltage of the transistor 22;

β₁₁ and β₃₄ represent the current amplification factors of thetransistors 11 and 34, respectively;

I_(E34) represents the emitter current of the transistor 34;

I₀ represents the constant current from the constant current source; and

R₃₃ and R₃₆ represent the resistance values of the resistors 33 and 36,respectively,

the base voltages V_(B11) and V_(B12) of the transistors 11 and 12 canbe expressed as follows: ##EQU1## If it is assumed that V_(B11) =V_(B12), the following equation is obtained: ##EQU2## where, if V_(BE22)= V_(BE34) and β₁₁ = β₃₄ are assumed in the above equation, thisequation can be expressed as follows:

    (R.sub.33 .sup.. I.sub.O)/2 = R.sub.36 I.sub.E34

    r.sub.33 .sup.. i.sub.0 = 2r.sub.36 .sup.. i.sub.e34

    r.sub.33 /r.sub.36 = 2i.sub.e34 /i.sub.0                   (1)

accordingly, if the values of the elements concerning the equation (1)are selected to satisfy the equation (1), V_(B11) = V_(B12) is obtained.Thus, even if the resistors 27 and 28 used in the circuit shown in FIG.2 are not provided in the embodiment of the invention shown in FIG. 3,in the invention there is no fear that any offset voltage is produced atthe terminals 19 and 21 and the differential amplifier 10 of theinvention operates as a linear amplifier. In the invention since theresistance value R₃₃ of the resistor 33 can be made great, the inputsignal applied to the input terminal 14 is not by-passed through theemitter electrode of the transistor 22. The base electrode of thetransistor 12 is by-passed through the emitter electrode of thetransistor 34. Further, since the resistors 27 and 28 used in the priorart are not used in this invention described as above, the voltage gainof this invention can be much increased.

As described above, with the differential amplifier according to thepresent invention, no offset voltage is produced at the terminals 19 and21 and the voltage gain of the differential amplifier 10 with thecircuit of the invention can be made great. Further, the circuit of theinvention readily can be formed as an integral circuit.

In the embodiment of the invention shown in FIG. 3, only the transistor11 is connected to the input terminal 14, but it may be possible toconnect further transistors to the input terminal 14.

FIG. 4 shows another embodiment of the invention in which the base ofanother transistor 38 is provided in connection with the input terminal14. The base electrode of the transistor 38 is also connected to theinput terminal 14, its collector electrode is connected to the voltagesource terminal 18 and its emitter electrode is grounded. The remainderof the circuit in FIG. 4 is substantially the same as that of FIG. 3,but, because of the fact that the input circuit of the transistor 38 isconnected in parallel with the input circuit of the transistor 11, thebase currents of both transistors 11 and 38 will flow through theresistor 33. This makes it necessary to determine the resistance valuessomewhat differently than in the circuit in FIG. 3.

Assuming that the emitter current of the transistor 38 is I_(E38) andthat the current amplification factor of the transistor 38 is β₃₈ andthat all of the other factors are as defined previously, the base biasvoltage V_(B11) on the transistor is now given by the equation: ##EQU3##As before, V_(B11) must be equal to V_(B12), which is still: ##EQU4##Assuming that the β's of the transistors 11, 34 and 38 are substantiallyequal and that the base-emitter voltages of the transistors 22 and 34are substantially equal, the following equation is obtained by settingV_(B11) = V_(B12) : or ##EQU5##

It will be apparent that many modifications and variations could beeffected by one skilled in the art without departing from the spirit orscope of the novel concepts of the present invention.

What is claimed is:
 1. A circuit comprising:A. a differential amplifiercomprising first and second differentially connected transistors, eachhaving an input electrode, said input electrode of said first transistorcomprising a signal input terminal for said differential amplifier; B. afirst emitter-follower bias circuit and first resistance meansconnecting said bias circuit to said input electrode of said firsttransistor; C. a second emitter-follower bias circuit connected to saidinput electrode of said second transistor; D. first means for supplyinga first predetermined bias voltage to a base circuit of said firstemitter-follower bias circuit; E. second means comprising secondresistance means for supplying a second predetermined bias voltage to abase circuit of said second emitter-follower bias circuit; and F. meansfor deriving an output terminal from said differential amplifier.
 2. Acircuit as cited in claim 1, wherein said first and second predeterminedbias voltage supply means respectively comprise a pair ofseries-connected resistors connected between a power source terminal anda reference potential terminal, said first resistance means comprising athird resistor connected in series between the base of said firstemitter follower bias circuit and a common circuit point between saidpair of resistors.
 3. A circuit comprising:A. an input terminal; B.differential amplifier including first and second base input electrodes;C. means for connecting said input terminal to said first inputelectrode of the differential amplifier; D. a first emitter-followerbias circuit comprising a first resistor connecting said bias circuit tosaid first input electrode of the differential amplifier; E. a secondemitter-follower bias circuit connected to said second input electrodeof the differential amplifier; F. first means for supplying apredetermined bias voltage to a base circuit of said firstemitter-follower type bias circuit; G. second means comprising a secondresistor for supplying said predetermined bias voltage to a base circuitof said second emitter-follower bias circuit; and H. means for derivingan output terminal from said differential amplifier.
 4. A circuit ascited in claim 3, wherein said differential amplifier comprises a pairof transistors and a common current source connecting both emitterelectrodes of said transistors to a reference potential terminal.
 5. Acircuit as cited in claim 3, wherein said predetermined bias voltage isgenerated by series-connected resistors which are connected between apower source terminal and a reference potential terminal.